This proposal is responsive to PA-06321, Cross-Disciplinary Translational Research At NIH (R21). Optimization of the capacity of monoclonal antibodies to mediate antibody-dependent cellular cytotoxicity (ADCC) should contribute to clinical activity. We are employing the techniques of antibody engineering to generate antibodies with improved ADCC properties that permit us to test two broad hypotheses: 1) antibodies with high affinity for a tumor antigen and for cellular Fc receptors will exhibit highly efficient ADCC and will mediate potent in vivo anti-tumor effects; and 2) such antibodies will induce adaptive immune responses that contribute to tumor efficacy. Preliminary data indicate that the capacity of recombinant anti-HER2/neu IgG antibodies to promote in vitro ADCC is related to their affinities for HER2/neu. Using other support we are preparing engineered human IgG1 monoclonal antibodies that bind to human HER2/neu and to Fc3R with varying affinities, to assess the in vivo anti-tumor effects of engineered human IgG1 monoclonal antibodies that bind to human HER2/neu and human Fc3RIII with varying affinities. Here we propose to test the hypothesis that treatment-induced in vivo ADCC promotes antigen presentation and initiates an adaptive immune response that contributes to tumor control and can be further amplified and prolonged by promoting antigen presentation, T cell costimulation and proliferation, and the expansion of antigen-specific cytotoxic T-cells. Specific Aim 1 is to determine the ability of engineered human IgG1 monoclonal antibodies that bind to human HER2/neu and to Fc3R to induce adaptive immune responses against human HER2/neu and to tumors expressing that antigen. The studies proposed here will lead to the creation of antibodies that mediate maximal ADCC and promote the induction of immune responses directed against autologous tumors in immunocompetent subjects. We have created an immunocompetent mouse strain that is transgenic for human HER2/neu to permit study of the link between ADCC and the induction of adaptive immune responses. Specific Aim 2 is to amplify monoclonal antibody-induced anti-tumor immune responses by combining anti-HER2/neu antibody therapy with anti-CTLA4, or other immunomodulatory approaches. This work will define the ability of passive antibody therapy to induce clinically relevant, adaptive anti-cancer immune responses and will define new avenues of treatment for patients with HER2/neu-positive tumors. The goal of this proposal is to use anti-HER2/neu monoclonal antibody therapy to stimulate immune responses against HER2/neu-expressing cancers. This work will identify strategies that amplify and sustain the anti-tumor immune response, and the results will be applicable to the general field of antibody therapy. A candidate antibody for clinical development will emerge, as will generally useful information regarding the structural features of antibodies that contribute to ADCC and in vivo efficacy. [unreadable] [unreadable] [unreadable]